1. Field of the Invention
The present invention relates to an AC power supply apparatus for use in applying a load to a cold cathode tube etc.
2. Description of the Related Art
Recently, a cold cathode tube is widely used as a light source for an LCD (liquid crystal display) unit etc. The adjustment of the amount of light of the cold cathode tube is performed depending on the amount of a current caused to flow through the cold cathode tube. Therefore, when there is a large amount of a current, it is bright and the power consumption is high. When there is a small amount of a current, it is dark and the power consumption can be low.
FIG. 9 shows a conventional AC power supply apparatus of a cold cathode tube in which a control signal is supplied together with a DC voltage to an input part 1, and the DC voltage is on/off controlled according to the control signal, thereby generating input power supply. An oscillator circuit 2 oscillates the power supply provided from the input part 1 at an AC frequency, boosts the voltage up to a predetermined voltage (for example, 600 V) by a booster circuit 3 configured by, for example, a transformer, and provides the result to a cold cathode tube through a connector 4.
Relating to the above-mentioned AC power supply apparatus, FIG. 10A shows a power supply waveform supplied from the input part 1 to the oscillator circuit 2. According to the control signal, the on/off control is performed every unit time, the ratio is controlled, and power is supplied to the oscillator circuit 2 on the basis of the ratio. The waveform shown in FIG. 10B is an output waveform of the oscillator circuit 2 to which an AC frequency is assigned by the oscillator circuit 2 in an energized period. The higher the frequency is, the higher the AC frequency tends to be in brightness.
As described above, with the conventional AC power supply apparatus, the energized/nonenergized time ratio shown in FIG. 10A is controlled, the current supplied to the cold cathode tube is varied, and the light modulation of the cold cathode tube is controlled.
However, in a notebook computer recently used in an LCD unit, a PDA (personal digital assistants), etc., the brightness of the display is controlled during, for example, transportation etc. such that it can be dark to reduce the power consumption. In this case, the energized period shown in FIG. 10A is controlled such that the period can be shorter. Although it is preferable that a high frequency is used with the brightness during use taken into account, a leakage current occurs in a cold cathode tube in this case if the energized period is shortened at a high frequency, and one end of the cold cathode tube becomes dark.
The patent document 1 discloses an invention of preventing a leakage current in the lighting circuit of a cold cathode tube used as a backlight of a liquid crystal display.
Patent Document 1: Japanese Published Patent Application No. H10-200174